Scheduled backups have stopped

[picaman]

Scheduled copies have stopped working for me. I have scheduled a backup of my internal hard drive to an external drive. Used to work fine but I noticed that the backing up had stopped. I deleted and rescheduled the 2am backup, and now the "Next copy" date changes every day but I never get a "Last copy" date populating, so no backups are happening. If I manually copy everything works fine.

I am logged in to the user account where I set the backup (the computer has two user accounts without admin privileges, and one admin account with them). The screen is set to sleep and lock with a password but the hard drive is set to never sleep.

Any ideas?